The aim of this project is to localize the MRI contrast agent gadolinium in cardiac and vascular tissues using electron probe x-ray microanalysis and electron energy loss spectroscopy (EELS). The sensitivity of the spectroscopic imaging approach is first demonstrated by mapping chelated gadolinium contained in sectioned polymer grains that are used to deliver the contrast agent. Experiments are now being performed to map the distribution of intracellular Gd in ischemic and control rat heart that has been treated with the gadolinium contrast agent. Tissue is rapidly frozen by clamping against liquid nitrogen cooled copper surfaces. Blocks are then transferred to an ultra-cryomicrotome where they are cut to a thickness of approximately 100 nm, picked up on copper grids coated with carbon-coated Formvar, and stored prior to analysis. It is hoped to use this approach to determine which subcellular compartments accumulate gadolinium. The specimens will be analyzed in a high-resolution STEM equipped with an energy-dispersive x-ray spectrometer.